The durations of pulses produced from mode-locked lasers can be as short as a few femtoseconds (1 fs=10.sup.-15 s) and typically have durations of less than 100 fs. The response times of the fastest electronic circuits are thousands of times longer than the duration of these pulses and therefore electronic techniques cannot be used to directly measure pulse durations. The shortest event available for measurement purposes is the pulse itself and this is the basis of optical autocorrelation techniques used for ultrashort pulse measurement.
In the most common autocorrelator arrangement, an input pulse, or parent pulse, passes into a Michelson interferometer which splits the parent pulse into two daughter pulses which are identical in shape and amplitude. The two daughter pulses travel along separate paths in the interferometer, one path being of variable length by use of a reflecting arm with a variable position. The two daughter pulses exit the interferometer overlapped spatially but with a relative temporal delay equivalent to the difference in path lengths travelled by each of the identical daughter pulses.
A two-wave mixing process, such as second-harmonic generation, is then used to obtain a mixing signal between the two daughter pulses. When the path lengths travelled by each daughter pulse are equal, the relative delay between the daughter pulses is zero and the mixing signal is strongest. As the difference in path length of the two daughter pulses increases, the product of the mixing decreases until, for time delays which are a few times longer than the pulse duration, the mixing signal becomes zero or at least insignificant. Therefore by studying how the mixing signal varies in response to changes in path length, a correlation signal can be obtained where width is related to the width, (i.e. duration), of the original input pulse.
Second-order autocorrelation, where the mixing signal varies quadratically with the optical input power, is common in mode-locked laser oscillators, and second-harmonic generation (SHG) has been used successfully to produce high-quality autocorrelation of sub-picosecond duration pulses. In SHG autocorrelation, the fields from each daughter pulse are coupled by a second-harmonic generation process to produce a wave at twice the fundamental frequency. The wave amplitude E.sub.2 is defined by: EQU E.sub.2 =E.sub.1a.times.E.sub.1b (1)
where E.sub.1a is the amplitude of a first daughter pulse, E.sub.1b is the amplitude of a second daughter pulse, and where both daughter pulses are directly derived from the same input parent pulse.
The second-harmonic intensity therefore varies quadratically with the input power to the Michelson interferometer. In practice, the further pulses which exit the Michelson interferometer are focused into a frequency-doubling crystal and the frequency-doubled light is then detected using a photomultiplier tube. The output voltage from the photomultiplier tube is then recorded as a function of the path difference between the pulses, or equivalently displacement of one of the interferometer arms, to give the autocorrelation signal of the input pulse.